Conventionally, an image forming apparatus such as a color printer includes a sheet conveying device that conveys a recording sheet (hereinafter referred to as sheet) stored in a paper feeding cassette to a printing position (a transfer position) in order to print (transfer) an image on the sheet. In the image forming apparatus that prints a toner image on the sheet, the sheet on which an unfixed toner image is transferred in the transfer position is conveyed to a fixing device. When the sheet passes a fixing nip portion, the unfixed toner image is heated and pressed to be fixed on the sheet.
The sheet conveying device conveys the sheet to the transfer position with registration rollers to be timed to coincide with the start of conveyance by the registration rollers such that a leading end position of a toner image born on an image bearing member such as a photoconductive drum or an intermediate transfer belt or a transfer belt and the leading end of the sheet are aligned. When the sheet is conveyed, the sheet is put on standby with the leading end thereof set in contact with a nip portion of the registration rollers not rotating. The photoconductive drum or the intermediate transfer belt is driven with process speed, which is speed for performing image formation, set as circumferential speed. The registration rollers are driven to be timed to coincide with the timing when the leading end of an image on the intermediate transfer belt moving at the process speed reaches to the transfer position, and convey the sheet from the registration position to the transfer position at the process speed. Then, the leading end of the toner image and the leading end of the sheet are aligned in the transfer position. The toner image is accurately transferred onto the sheet.
When printing on plural sheets or printing of continuous pages is performed, image formation is performed with continuously fed sheets spaced apart from each other by an inter-sheet distance. Therefore, if the registration rollers are restarted at the process speed at the timing for starting registration, sheets are conveyed to the transfer position in printing after that while the inter-sheet distance is kept.
In the color image forming apparatus, for example, color toners of four colors, yellow (Y), magenta (M), cyan (C), and black (K), are born in order on the image bearing member such as the transfer belt. A sheet on which unfixed color toner images are transferred in the transfer position is conveyed to the fixing device and fixed on the sheet by pressing and heating. When a color image is continuously printed, it is desirable to set the inter-sheet distance as short as possible in order to secure fixability of the color toners. Further, if color printing speed (PPM: print per minute) is reduced, a heat quantity per one sheet can be secured. In this case, it is desired to realize a reduction in the inter-sheet distance and realize an increase in speed as much as possible, although the number of prints decreases.
As an image forming apparatus including a sheet conveying device that aligns the leading end of a toner image on an image baring member and the leading end of a sheet according a reduction in an inter-sheet distance, there is an image forming apparatus disclosed in JP-A-59-24870.
The image forming apparatus including the sheet conveying device in the past horizontally feeds a sheet in a paper feeding cassette directly to registration rollers with a paper feeding roller and horizontally conveys the sheet to a transfer position for transferring a toner image. The start of second and subsequent sheets by the registration rollers is controlled according to the registration start timing. An inter-sheet distance of sheets conveyed by the registration rollers to the transfer position at process speed is set as a normal inter-sheet distance. When the inter-sheet distance is set shorter than the normal inter-sheet distance, if the registration rollers are restarted at the registration start timing, at the time of the restart, a leading end position of the toner image is present further on the transfer position side than that in the case of the normal inter-sheet distance. Therefore, registration acceleration for conveying the sheet at sheet conveying speed by the registration rollers higher than the process speed is performed. When the leading end of the sheet catches up with the leading end of the toner image on the image bearing member that moves at the process speed, the sheet conveying speed by the registration rollers is reduced to the process speed to align the leading end of the toner image and the leading end of the sheet in the transfer position.
In the sheet conveying device in the past, the sheet fed from the paper feeding cassette to the registration rollers by the paper feeding roller is present in the paper feeding cassette and comes into contact with the paper feeding roller in a standby state in which the leading end of the sheet is in contact with the registration rollers. When the sheet conveying speed of the registration roller is reset to the process speed from the speed higher than the process speed, the sheet is still in contact with the paper feeding roller.
Therefore, after the sheet is fed to the registration rollers, driving to the paper feeding roller is disconnected and the paper feeding roller is idled by frictional contact with the sheet conveyed by the registration rollers to make it possible to convey the sheet. However, a load of sheet conveyance involved in the idling of the paper feeding roller is excessively applied to the registration rollers. Therefore, it is necessary to increase the power of a registration motor that drives the registration rollers.
However, when the sheet is conveyed by the registration rollers, because of the influence of a change in rigidity of the sheet due to the basis weight (thickness) of the sheet, an environment such as temperature and humidity, or the like, friction with a guide surface of a sheet conveyance guide increases, which becomes a load on sheet conveyance. This sheet conveyance load further increases according to an increase in the sheet conveying speed and causes a loss of synchronism of the registration motor and the conveying motors on the way.
If the speed of the registration acceleration is reduced to reduce the registration speed to the process speed immediately before transfer, the sheet conveyance load can be reduced. However, fluctuation in a load involved in the speed change occurs in the sheet and causes a blur and, in the case of a color image, causes color drift or the like.
On the other hand, in an image forming apparatus in which paper feeding cassettes are vertically arranged in plural stages, registration rollers are arranged on a sheet conveying path extending in an up to down direction and conveying rollers are arranged below the registration rollers to correspond to the paper feeding cassettes in the respective stages. A sheet in any one of the paper feeding cassettes is fed by the paper feeding roller to the conveying rollers corresponding thereto, conveyed to the registration rollers at conveying speed higher than process speed, and put on standby for restart for driving the registration rollers, which are temporarily stopped to drive sheet conveyance, again to perform sheet conveyance. In this case, depending on sheet length (sheet size) and a stage position of the paper feeding cassette, the sheet is nipped by at least the conveying rollers immediately below the registration rollers and waits for the restart of the registration rollers.
A driving mechanism for the conveying rollers and the paper feeding rollers includes a clutch mechanism. The driving mechanism turns off the clutch mechanism to thereby disconnect power transmission from driving motors to the conveying rollers and the paper feeding roller to allow the rollers to idle. In this case, even if it is attempted to convey the sheet to an upper transfer position with only the registration rollers according to a sudden change in speed such as the registration acceleration, stable sheet conveyance control is difficult because of the influence of the conveyance load and the like. Therefore, under the present situation, highly accurate positioning and conveyance of the sheet cannot be performed.